Do soot emissions mean that wood heating causes global warming?
Tuesday, 19 January 2010 00:00
Soot is made up of tiny carbon particles that result when fuels are burned incompletely. The puff of black exhaust smoke from a diesel truck or bus, and the black stains on exhaust pipes and chimney caps are obvious evidence of soot emissions. The role of soot in global warming only came to public attention in the past ten years and the research into its exact effects is still in its infancy. But the issue is being confused by various anti-wood burning activists who use the breaking news about soot and global warming to argue that using wood energy actually causes global warming, contrary to the widely accepted idea that wood energy is carbon neutral. A little explanation is needed to show how very wrong these activists are when they make that claim. In the process we gain insights into how scientific data can be used selectively to support a particular agenda. "Soot particles containing black carbon, from fossil-fuel and biofuel [wood, brush, dung, ethanol and bio-diesel] burning sources, have a strong probability of being the second-leading cause of global warming after carbon dioxide and ahead of methane", says Mark Jacobson, Professor of Civil and Environmental Engineering at Stanford University and one of the foremost researchers in the field. By far the largest regional source of soot is brush fires and biomass burning in China, India and other parts of Asia, accounting for between 25 and 35 percent of global soot emissions, according to soot emission specialists. The soot emitted in developing nations results from the burning of field stubble and the estimated 2.5 billion people who cook their food on open fires. Dr. Tami Bond of the University of Illinois, Urbana Champaign, estimates the sources of black carbon emissions as follows: 42% Open biomass burning (forest and savanna burning) 18% Residential biofuel burned with traditional technologies 14% Diesel engines for transportation 10% Diesel engines for industrial use 10% Industrial processes and power generation, usually from smaller boilers 6% Residential coal burned with traditional technologies Jacobson goes on to say that "About half of the U.S. black carbon in particles smaller than [PM2.5] is from fossil fuel sources. The rest is from area sources; agricultural fires, structural fires, slash/prescribed burning forest wildfires, unpaved road dust, paved road dust, and construction dust, according to the 2002 U.S. National Emissions Inventory." He doesn't even mention wood as an energy source. Why?, because it represents a tiny fraction of U.S. soot emissions, much less the global total. The use of wood fuel is widely considered to be "carbon neutral" because trees absorb carbon dioxide when they grow and release about the same amount back to the atmosphere whether they are burned for heating or die and rot in the forest. It follows, therefore, that when firewood is used to displace the burning of fossil fuels, a net reduction in greenhouse gases results. Although wood heating is not strictly carbon neutral because of fossil fuel inputs to firewood production and transportation, and because some by-products of combustion such as methane and soot have global warming potential, on balance wood still has a far lower global warming impact than the fossil fuels oil, gas and coal. Political strategists preach that it is more effective to attack your opponent's strengths than to focus on their weaknesses. That is why the anti-wood burning web sites and commentators have seized on the issue of black carbon emissions to argue that burning wood to heat homes causes global warming. To make this argument, however, they have misinterpreted the available scientific data. This page, titled Why burning wood is not 'green' or 'Carbon Neutral', is a listing of articles purporting to challenge the claim that wood heating in a North American context is carbon neutral. Even though wood energy as used in North America is a very small contributor to total soot emissions, it is still important to minimize all causes of global warming. Advanced technology, EPA certified, wood heaters cut emissions from an average of 25 to 40 grams of particulates per hour to an average of 2 to 5 g/h. Soot is a component of these particulate emissions. According to V. Ramanathan of the Scripps Institution of Oceanography at the University of California at San Diego, another of the main researchers into the role of black carbon (BC) in global warming, "In the atmosphere, black carbon aerosols are mixed with sulfates and organics and it is not straight forward to untangle the effect of black carbon from that of the mixed (black carbon and others) aerosol. Thus most if not all of the published estimates of black carbon are derived from models." While there is little dispute that BC has a climate-changing effect, there continues to be uncertainty about several aspects such as the total amounts, their distribution and relative effects. Currently it is not possible to be definitive about the exact contribution that soot from residential wood heating has to its global warming potential. It is clear, however, that as practiced in North American and using EPA certified appliances, wood heating has lower global warming potential than any other fuel source. JG
Source : The woodpile
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